CSS Sentry

Browser extension for detecting and reducing CSS-based data exfiltration risk.

CSS Sentry is a local-only browser extension that helps detect and reduce risk from CSS-based data exfiltration techniques. It analyzes page styles for suspicious selector probing, remote CSS resource sinks, CSS imports, SVG/CSS resource behavior, data stylesheets, and rendered-content CSS injection patterns. The extension is designed as a defense-in-depth tool. It includes Passive, Balanced, and Strict protection modes, local reports, destination allow/block policy support, advanced compatibility options, and optional Firefox-specific enhanced stylesheet inspection. CSS Sentry does not use telemetry, cloud analysis, or a remote service. Reports are stored locally in your browser. This extension does not claim to prevent every CSS side channel, browser bug, sanitizer bypass, or future CSS feature. It is intended to reduce exposure to known high-signal CSS exfiltration patterns while minimizing normal browsing breakage.